1. Field of the Invention
The present invention relates generally to casting apparatus and, more particularly, to certain new and useful improvements in apparatus and methods for the automatic casting of precision castings such as dental prostheses, jewelry and small precision parts.
2. Description of the Prior Art
Dental castings, such as dentures, crowns and bridges, are a frequent example of castings requiring great precision to fit properly. Typically, heretofore such castings have been made by the use of the lost wax process with a mold constructed of a porous investment. The investment has a cavity which is to be filled with molten metal such as a gold or nickel alloy. An entry port to the cavity may be formed as a hemispherical depression in the outer surface of the investment. The entry port is connected by sprues to the cavity. The following procedure has been used. An ingot of the metal to be cast is first placed in a separate crucible and heated as by means of a blow torch or induction coil. Thereafter, the molten metal is forced to flow from the crucible into the entry port of the mold, and then via the sprues into the cavity by centrifuging the crucible together with the mold. Where heating is done by an induction coil, the coil is placed around the crucible at a distance from the mold since the iron ring, or flask, which has often been used for enclosing the investment would deter the electromagnetic energy from reaching the ingot. Such a heating, or melt, coil is removed from the crucible prior to the centrifuging. The speed of flow of the metal must be high enough to fill the mold completely prior to freezing, or solidification, of the metal along the surfaces of the sprues and the cavity. Entrapped air in the cavity may exit through a vent sprue or through the pores of the investment material under the pressure of the advancing metal. The investment material is typically a refractory ceramic such as silica held together by a binder such as gypsum.
As heretofore practiced, a problem has arisen in that, during the heating of the metal ingot, impurities from the atmosphere and from the heating flame are absorbed into the molten metal and carbon from the heating flame further contaminates the resulting castings. A further problem has arisen in castings made with lower density metals wherein the resulting forces of a centrifuge are reduced, increasing the chances of voids in the casting due to the reduced forces. Alternatively, as a result of the reduced forces available, it has also heretofore been a common practice to utilize an excess amount of molten metal, often as much as twice that required for the desired cast, in an attempt to insure that the desired amount of molten metal is forced into the mold cavity. This practice is economically disadvantageous from a materials standpoint and also because it leaves oxide impurities on the crucible surface which must be removed. Remelting and reuse of the excess metal is also both economically disadvantageous and undesirable.
While early workers in the casting field heated the metal by means of a torch directly in the entry port to the mold sprue system, the entry port serving as a crucible, the more recent practice has been to heat the metal at a remote crucible, often to temperatures in excess of the melting temperature, followed by a rapid delivery of the molten metal to the mold as by use of the centrifuge. Although the earlier practice is preferable in that less heat is lost during transfer of the metal from the crucible to the mold cavity, that practice suffered a disadvantage in that the metal flasks which contained the investment would not admit the use of induction heating which could be employed at the remote crucible. Present day investment materials do not require the metal flask for adequate mold strength. In addition, both procedures are often practiced by the use of temperatures substantially in excess of the melting temperature of the metal, which is harmful to the properties of the casting material.
It is known that the effects of thermal expansion/contraction between the casting and the mold can be compensated for by preheating the mold, whereby, upon cooling of the casting, dimensional accuracy is assured. However, in this respect, another problem has arisen in the foregoing casting processes in that independent control of the mold temperature and the metal temperature cannot be provided simultaneously. As previously practiced, whether the metal is heated at the crucible or remote from the crucible, it has not been possible to maintain heating of the metal during passage through the sprues and within the cavity so as to retard solidification until the cavity is completely filled.
3. Objects of the Invention
It is therefore an object of this invention to provide novel and improved apparatus and methods for automatic precision casting.
Another object of this invention is to provide novel and improved apparatus and methods for automatic precision casting which eliminate or overcome the shortcomings and disadvantages of previously known casting apparatus and methods.
Another object of this invention is to provide novel and improved apparatus and methods for automatic precision casting which obtain castings of greatly improved quality with greatly improved reliability and economic advantages.
Another object of this invention is to provide novel and improved apparatus and methods for automatic precision casting which eliminates the need for a separate melting crucible or a metal casing ring surrounding the investment material of the mold and permits casting with both the molten cast material and the investment material maintained at lowered temperatures.
Objects and advantages of the invention are set forth in part herein and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.
The invention consists of the novel parts, constructions, arrangements, combinations, steps, processes and improvements herein shown and described.